Aromatic carboxylic acids, such as benzoic acid, phthalic acid, terephthalic acid, trimethyl benzoic acids, naphthalene dicarboxylic acids and the like, are used widely as intermediates in the chemical industry. Aromatic carboxylic acids are prepared by oxidation of their corresponding alkyl aromatic compounds (see Suresh, A., “Engineering Aspects of Industrial Liquid-Phase Air Oxidation of Hydrocarbons,” Ind. Eng. Chem., Vol. 39: p. 3958-3997, (2000)). For instance, terephthalic acid is prepared by oxidation of p-xylene, as shown in the schematic below:

Terephthalic acid, TPA (1,4-benzenedicarboxylic acid), is of commercial interest to the polymer industry because of its use in the manufacture of saturated polyesters, such as polyethylene terephthalate (PET), 1,2-ethanediol, and copolymers thereof. Worldwide production of TPA and its corresponding dimethyl ester, dimethyl terephthalate, ranked about 25th in tonnage of all chemicals produced in 1992, and about 10th of all organic chemicals.
As shown in the scheme below, the oxidation of p-xylene is a radical initiated, step-wise reaction which produces two main intermediates, p-toluic acid and 4-formyl-benzoic acid.

Incomplete oxidation of 4-formyl-benzoic acid (4-CBA) leads to contamination of TPA purity. Removal of 4-CBA is complicated by the fact that it co-crystallizes with TPA due to its structural similarity with TPA. Contamination with 4-CBA can be substantial; for instance, there are production processes that yield a TPA stock which have approximately 5000 ppm of 4-CBA (Perniconea et al., “An investigation on Pd/C industrial catalysts for the purification of terephthalic acid,” Catalysis Today, Vol. 44: p. 129-135 (1998)). Thus, subsequent purification steps after TPA production are often necessary in order to attain TPA feedstock of sufficient purity for high-grade polyester synthesis (see Matsuzawa, K. et al., “Technological Development of Purified Terephthalic Acid,” Chemical Economy & Engineering Review, Vol. 8 (9): p. 25-30 (1976)).
There are numerous process methods available for manufacturing TPA, each of which have varying production and purity yields for TPA. Most of these processes involve oxidation of p-xylene with an oxygen source e.g. air or O2 gas, in the presence of liquid phase, homogeneous catalysts containing at least cobalt and/or manganese metals. In addition, most of these processes are conducted in the presence of an acidic solvent, such as acetic acid, and as a rule employ corrosive bromine promoters as a radical source e.g. HBr, NaBr, or other metal bromines. Thus, these processes are typically conducted in expensive, titanium-clad reactors that can accommodate such harsh reaction conditions. Representative methods for manufacturing TPA are described in the following patents and publications, the disclosures of all of which are incorporated herein by reference.
U.S. Pat. Nos. 2,833,816 and 3,089,906 report a process for oxidizing a polyalkyl aromatic compound with O2 in acetic acid solvent using a metal bromine catalyst.
U.S. Pat. No. 4,786,753 reports a process for oxidizing di- and trimethyl benzenes in the presence of an aliphatic acid in the presence of a nickel, zirconium, and manganese catalyst system with a bromine source.
U.S. Pat. No. 4,877,900 report a two-stage oxidation process for p-xylene with molecular oxygen in the presence of a heavy metal catalyst and bromine, wherein the second stage involves post-oxidation with molecular oxygen and is conducted at a higher temperature then the first stage.
U.S. Pat. No. 4,892,970 reports a two-stage process for the oxidation of alkyl benzenes in the presence of a cobalt, nickel, or zirconium metal catalyst and bromine, wherein additional bromine is added to a second stage of the process.
U.S. Pat. No. 5,453,538 reports a process for oxidizing dimethyl benzene with molecular oxygen in a C1-C6 aliphatic carboxylic acid solvent with a cobalt, manganese, and cerium catalyst and a bromine source.
U.S. Pat. Nos. 5,596,129 and 5,696,285 reports a process for oxidizing alkyl benzenes by supplying a nearly pure O2 gas source to the reactor. These processes are conducted in an acetic acid/water medium and utilizes a cobalt, manganese, and bromine catalyst.
Cincotti, A. et al. (“Kinetics and related engineering aspects of catalytic liquid-phase oxidation of p-xylene to terephthalic acid,” Catalysis Today, Vol. 52: p. 331-347, (1999)) reports a kinetic model for TPA production. This study evaluated the oxidation of p-xylene in a methyl benzoate solvent using cobalt naphthenate as a catalyst. P-tolualdehyde was used as a promoter source and either pure oxygen or air was the oxidation source.
Dunn, J. et al. (“Terephthalic Acid Synthesis in High-Temperature Liquid Water, Ind. Eng. Chem. Res., Vol. 41: p. 4460-4465, (2002)) reports a TPA synthesis process in liquid water at temperatures ranging from 250 to 300° C. This process utilizes hydrogen peroxide, instead of air or O2, as an oxidant.
The following catalysts were evaluated in the study: manganese bromide, cobalt bromide, manganese acetate, nickel bromide, hafnium bromide, and zirconium bromide.
Partenheimer, W. et al., (“The effect of zirconium in metal/bromide catalysts during the autoxidation of p-xylene,” Journal of Molecular Catalysis A: Chemical, Vol. 206: p. 105-119, (2003)) reports the oxidation of p-xylene in acetic acid medium with a zirconium catalyst and either a cobalt, manganese/bromide, nickel/manganese/bromide, or cobalt/manganese/bromide catalyst.
The less corrosive bromoanthracenes, in comparison to NaBr or HBr, have been employed as a bromide source in the oxidation of p-xylene. Saha et al. (“Bromoanthracenes and metal co-catalysts for the autoxidation of para-xylene,” Journal of Molecular Catalysis A: Chemical, Vol. 207: p. 121-127, (2004)) reports the oxidation of p-xylene in acetic acid using 9,10-dibromoanthracene or 9-bromoanthracene in the presence of Co(OAc)2 and either a Mn(OAc)2, Ce(OAc)3, or ZrOCl2 co-catalyst.
Methods for TPA manufacturing that use solid catalysts include Chavan et al. (“Selective Oxidation of para-Xylene to Terephthalic Acid by μ3-oxo-bridged Co/Mn Cluster Complexes Encapsulated in Zeolite-Y,” Journal of Catalysis, Vol. 24: p. 409-419, (2001)) and Srinivas et al. (U.S. Pat. No. 6,649,791 and U.S. Patent Application Publication No. 2003/0008770). In these methods, solid catalysts of μ3-oxo-bridged Co/Mn cluster complexes, [Co3(O)(CH3COO)6(pyridine)3]+, [Mn3(O)(CH3COO)6(pyridine)3]+, and CoMn2(O)(CH3COO)6(pyridine)3, are encapsulated in Zeolite-Y and the oxidation process was carried out in an acetic acid/water solvent using NaBr as a radical initiator.
TPA has also been prepared employing a solid catalyst without the use of bromide ions. Jacob et al. (Journal Applied Catalysis A: General, Vol. 182: p. 91-96, (1999)) described the aerial oxidation of p-xylene over Zeolite-encapsulated salen, saltin, and salcyhexen complexes of cobalt or manganese using t-butyl hydroperoxide as the initiator. This process converts up to 50-60% of p-xylene; however, the yields of TPA are low and the main product attained is p-toluic acid.
Currently, there exists a need for methods of synthesizing aromatic carboxylic acids with sufficiently high yields and suitable purity for subsequent high-grade manufacturing processes, so as to obviate the need for additional purification steps. In addition, there exists a need for methods that avoid the use of corrosive feed materials or other process materials which may be harmful to the environment, such as acetic acid, NaBr, or HBr.